Thank you very much for your reply. I did not really found what I was looking for in the thread your referred me to. However, I had already heard about Spootnik and found a (costly) solution there. Here it is for the benefit of other forums visitors:
In order to meet my needs (listed above):
- I need create a Basecamp account (free) and to add my colleagues to it.
- Then, we must each have a Spootnik account (12$/month each) and link our OmniFocus with it.
- Finally, we must each link our Spootnik account to the Basecamp project.
We tested it and it worked.
If anybody finds a better way to do it, please share!